Superporous hydrogels (SPHs) are three-dimensional network hydrophilic polymers that absorb a considerable amount of water in a very short period of time due to the presence of many pores having diameters on the micron to millimeter scale (1-4). They are distinguished from other porous hydrogels in terms of their pore sizes and the methods used to generate the pores. For example, the generally accepted sizes for microporous, mesoporous, and macroporous hydrogels are in the range of 10 to 100 nm, 100 to 1000 nm and 1 to 100 μm, respectively. freeze drying (5), porogens (6-8), microemulsion (9) and phase separation methods (10). In contrast, SPHs are normally prepared by a gas blowing technique, such as is employed in the preparation of plastic foams, e.g., polystyrene or polyurethane foams (11, 12). Pores are generated by the introduction of a small amount of blowing agent within the SPH formulation, which can create gaseous volatile materials as the reacting mixture gels. Faster and greater water absorption are achieved due to the presence of the pores within the SPH structure. Representative reports of such methods and compositions are presented by Park et al. in U.S. Pat. Nos. 5,750,585 and 6,271,278.
The fast and high water absorbent properties of SPHs afford potentially many industrial applications in agriculture, horticulture, hygiene, construction, medical and biomedical fields. They can be used as a particulate to impart a well-defined shape to water-absorbent polymers. They can meet the requirements of such applications as long-term water holding in horticulture and hygienic products, water sealing, or caulking in civil constructions, and the like.
Although previous hydrogel foams and SPHs clearly have industrial applications, a significant shortcoming needs to be overcome in order to extend their applications. As the SPHs absorb lots of water, they become mechanically too weak to maintain their functions for long periods, with their physical structure being easily destroyed even under small stresses. They are rarely re-used once contacted with water. These problems are attributed to the inherent low elasticity properties of SPHs when swollen in water.
Accordingly, it is desired to develop SPHs having enhanced mechanical strength, e.g., by increasing their elasticity, in order to sustain their functions even under rather severe conditions. It is expected that mechanically strong or elastic superporous hydrogels can be used in a vast variety of applications, including those previously reported by Park et al. in U.S. Pat. Nos. 5,750,585 and 6,271,278.